Hello,
There are two main reasons for this difference:
For starters, AToW offers players a variety of different modifiers and features that are simply not accounted for in a Total Warfare setting, where your "character" is nothing more than two numbers reflecting his Piloting and Gunnery Skills. For one, the amount of damage that constitutes a Consciousness modifier varies with BOD (A character with a BOD of 2 should rightly be sleeping once he's hit at all, while a character who has a BOD of 8 should not consider a KO-inducing hit at BOD 2 much more than a stunning blow--albeit still one that can drop him if he's unfortunate enough). For another, the character's WIL Link Attribute modifier also applies to the Consciousness Check, allowing strong-willed characters a better advantage. Total Warfare also doesn't account for Toughness, Glass Jaw, Pain Resistance, and so on. These features make for much more variety in gameplay options--and that's before we remember that--as briefly mentioned in the equipment section--LSSU (Medipacks) are something of a standard item in MechWarrior cockpits (they're not standard issue, but they do get mentioned a lot in fiction).
(Worth noting, BTW, is that the damage caused by various pilot-damaging actions in TW is not based on your character's size, but is very basic. Whether the character is BOD 2 or BOD 8 does not affect the damage taken in a single head hit, nor is such a hit automatically "one fifth" of said character's damage capacity, to match TW, as that was considered way too arbitrary.)
The second reason is a bit more critical: Based on the presumption that any character you cared enough to create is likely a character you'd like to keep a while longer, KO is encouraged sooner, to avoid keeping them awake and battle-ready longer than they should. In Total Warfare, it is rather common for a player to decide on an Alpha Strike, a suicidal charge, or some other form of potentially life-ending insanity as long as the pilot is nothing more than a pair of Skill numbers. A fully fleshed out character, however, is a lot deeper and should be more valuable to the controlling player.
Hopefully, that helps.
Thank you,
- Herbert Beas
BattleTech
Catalyst Game Labs